Cis-trans isomerism

- [Voiceover] Let's say we
were asked to name the molecule

on the top left.

We would start by numbering our carbons.

So this would be carbon
one, two, three, and four.

Notice we have a double
bond starting at carbon two.

So the name of this
molecule would be 2-butene.

Two because we have our double
bond starting at carbon two.

"But" because we have four
carbons and "ene" because

we have a double bond
presence in the molecule.

What about naming the
molecule on the right?

We number our carbons
one, two, three, and four.

And once again we have a double
bond starting at carbon two.

So the name of this
molecule would be 2-butene.

However these are two
different molecules and

the reason why is because
there's no free rotation

around a double bond.

Single bonds have free rotation
but double bonds don't.

So you couldn't rotate the
molecule on the left to

look like the molecule on the right.

Therefore they must be
isomers of each other

and we need a way to
distinguish between our isomers.

And so one way to do that is
to use cis/trans terminology.

So if we look at the molecule on the left,

we can see we have two methyl groups.

And those two methyl
groups are on the same side

of our double bond.

So if I draw a line in here,
it's easier to see those two

methyl groups are on the same sides.

And we call that the cis isomer.

So we put cis in front of our name here.

I'm attempting to write it in italics.

So this would be cis-2-butene.

On the right when we look
at those methyl groups,

these two methyl groups
are on opposite sides

of the double bond.

So I draw a line in here
to make it easier to see

those two methyl groups
are on opposite sides.

And we call that trans.

So this is trans isomer.

I'm going to write trans here
in italics, attempt to anyway.

So we have cis-2-butene
and trans-2-butene.

These are different molecules
with different properties.

If you want to use cis/trans terminology,

you're looking for two identical groups

and you are comparing them.

So let's look at these next two
examples here and figure out

which one is cis and which one is trans.

We're looking for identical groups.

So over here we have
an ethyl group attached

to our double bond

and on the right we have an
ethyl group to our double bond.

Those two ethyl groups
are on the same side of

our double bond so this
must be the cis isomer.

On the right we have this ethyl
group and this ethyl group

on opposite sides of our double bond.

So that must be the trans isomer.

All right, let's do some more examples.

I'll go down to here.

On the left we have
cinnamaldehyde molecule.

We're looking for two
identical groups so we can use

cis or trans.

You can also use hydrogens, right.

You don't have to use a
methyl group or an ethyl group

so if we look at our double
bond we know there's a

hydrogen attached to this carbon

and we know there's a hydrogen
attached to this carbon.

And those two hydrogens are
on opposite sides of our

double bonds.

And I'm drawing a line here
to make it easier to see.

Right, these two hydrogens
are on opposite side

so we're talking about trans here.

Those hydrogens are
across from each other.

What about the tetra-substituted alkene

on the right?

We need two identical
groups to use our cis/trans

and here we have an ethyl
group, and here we have

an ethyl group.

All right over here we
have a methyl group and an

isopropyl group.

But the two methyl groups
are on the same side of

our double bond.

So I draw a line in here and
we see that these two groups

are on the same side,

therefore we're talking about cis here.

So this double bond has
a cis configuration.

Let's compare the drawing
on the left to the

drawing on the right.

The first time you look
at these two drawings

you might think these are two isomers,

and I could use cis/trans
terminology to distinguish

between them.

However, you can't because these are

just two ways to represent
the same molecule.

If you picked up this
molecule on the left and

you flipped it up,

you would get the drawing on the right.

So they're not isomers of each other.

This is the same molecule.

And a fast way to figure that
out is to look at this carbon.

And you can see you have
two identical groups bonded

to that carbon.

So you can't use cis/trans terminology.

That's different from the
example we did a minute ago.

We had two identical groups,

right these two ethyl groups here.

However those two ethyl
groups weren't bonded

to the same carbon.

Those two ethyl groups are
bonded to different carbons.

So this ethyl group is bonded
to this carbon, and this

ethyl group is bonded to this carbon.

So we were able to use
cis/trans terminologies.

So we looked at our
double bond and we said

those two ethyl groups
are on the same side of

our double bond,

so this represents a cis
configuration of the double bond.

So we can't do that up here because

while we do have two identical groups,

those identical groups are
bonded to the same carbon.